This invention generally relates to the field of mechanical refrigeration, and more particularly to energy-saving compression-type refrigeration systems.
In the operation of commercial freezers, refrigerators, air conditioners, and other compression-type refrigeration systems, it is desirable to maximize refrigeration capacity while minimizing total energy consumption. In addition, it is necessary to suppress the formation of "flash gas." Flash gas is the spontaneous flashing or boiling of liquid refrigerant resulting from pressure losses and frictional heating in refrigerant lines. Various techniques have been developed to eliminate flash gas. However, conventional methods for suppressing flash gas can substantially reduce system energy efficiency.
FIG. 1 represents a conventional mechanical refrigeration system 10 of the type used in a supermarket freezer. Specifically, compressor 12 compresses refrigerant vapor and discharges it through line 14 into condenser 16. Condenser 16 liquifies the refrigerant, which next flows through lines 20a and 20 into receiver 22. From receiver 22, the liquid refrigerant flows through line 26 to counter-current heat exchanger 28. After passing through exchanger 28, the refrigerant flows via line 29 through thermostatic expansion valve 30. Valve 30 expands the liquid refrigerant into a vapor which flows into and through evaporator 34. Valve 30 is connected to thermostat 32 by lead wire 31. Thermostat 32 throttles valve 30 to regulate temperatures produced in evaporator 34 by the expanded vapor. Passing through evaporator 34, the expanded refrigerant absorbs heat, aided by circulating fan 38, and then returns to compressor 12 through line 40.
To suppress flash gas, the refrigerant temperature at condenser 16 is conventionally maintained at approximately 95.degree. F. Pressure levels in receiver 22 are conventionally maintained above the flash or boiling point of the refrigerant; 125 PSI for R12 refrigerant, 185 PSI for R22 refrigernt, and 185 PSI for R502 refrigerant. These temperature and pressure levels are sufficient to suppress flash gas formation in lines 26 and 29, but conventional means for maintaining such levels degrade system efficiency.
Various means are used to maintain the temperature and pressure levels stated above. For example, FIG. 1 shows a fan unit 18 connected to sensor 15 in line 20a. Controlled by sensor 15, fan unit 18 is responsive to condenser temperature or pressure and cycles on and off to regulate condenser heat dissipation. A pressure-responsive bypass valve 19 in condenser output line 20a is also used to maintain pressure levels in receiver 22. Normally, valve 19 is set to enable a free flow of refrigerant from line 20a into line 20. When the pressure at the output line of condenser 16 drops below a predetermined minimum, valve 19 operates to permit compressed refrigerant vapors from line 14 to flow through bypass line 14a into line 20. The addition to vapor from lines 14 and 14a into line 20 increases the pressure in receiver 22, line 26, and line 29, thereby suppressing flash gas.
The foregoing system eliminates flash gas, but is energy inefficient. First, maintaining a 95.degree. condenser temperature reduces compressor capacity and increases energy consumption. Although the 95.degree. temperature level maintains sufficient pressure to avoid flash gas, the resultant elevated pressure in the system produces a back pressure in the condenser which increases conpressor work load. The operation of bypass valve 19 also increases back pressure in the condenser. In addition, the release of hot, compressed vapor from line 14 into line 20 by valve 19 increases the specific heat in the system. The added heat necessitates yet a higher pressure to control flash gas formation and reduces the cooling capacity of the refrigerant, both of which reduce efficiency.
Another approach to suppressing flash gas has been to cool the liquid refrigerant to a temperature substantially below its boiling piont. As shown in phantom line in FIG. 1, a subcooler unit 42 has been used in line 26 for this purpose. However, subcooler units require additional machinery and power, increasing equipment cost and reducing overall operating efficiency.
Other methods for controlling the operation of refrigeration systems are disclosed in U.S. Pat. Nos. 3,742,726 to English, 4,068,494 to Kramer, 3,589,140 to Osborne, and 3,988,904 to Ross. For example, Ross discloses the use of an extra compressor to increase the pressure of gaseous refrigerant in the system. The high pressure gaseous refrigerant is then used to force liquid refrigerant through various parts of the system. However, each of these system is complex and requires extensive purchases of new equipment to retrofit existing system. The expenses involved in these purchases usually outweigh the savings in power costs. Thus, none of the above patents provide a simple, low-cost method of eliminating flash gas without extensive system modification. Also, they do not appear to maximize refrigeration capacity.
Accordingly, a need remains for an effective way to suppress flash gas in compression-type refrigeration systems without impairing refrigeration capacity and efficiency.